1. Field of the Invention
The present invention relates generally to unloaders for reciprocating gas compressors, and in particular to an unloader system that allows variable use of fixed or variable clearance volumes.
2. Description of the Related Art
Gas compressors are well known and various types have been utilized to meet the requirements of particular applications. For example, natural gas transmission through pipelines is often accomplished with large, reciprocating compressors driven by internal combustion engines at pumping stations located along the pipeline routes.
In natural gas transmission, the internal combustion engines which drive the compressors are often fueled by natural gas taken directly from the pipeline. Thus, the fuel consumed by the engines driving the compressors reduces the overall operating efficiency since the amount of gas delivered is reduced by amounts consumed in the transmission or pumping process.
Efficient operation of natural gas compressors typically involves the use of a computerized control system for controlling the air/fuel mixture, rotational speeds, etc. Another factor which has a significant effect on compressor operating efficiency relates to the extent to which the compressor is loaded. In fully-loaded operation, the maximum output of the compressor is achieved, with a resultant full load on the compressor engine. However, natural gas compressor flow demands can vary considerably, and typically depend on downstream demand factors and conditions.
Controlling compressor flow is often accomplished by partially xe2x80x9cunloadingxe2x80x9d a compressor whereby each compressor stroke produces a reduced gas flow as compared to fully-loaded operation. Reduced gas flow generally corresponds to reduced work performed by the compressor engine, whereby fuel savings and greater efficiency can be achieved. Although compressor output could be varied by changing the speed of the driving engine, this approach is often impractical because the engines are designed to operate at constant speeds for maximum fuel efficiency and minimum emissions. Thus, compressor output control must normally be accomplished using other means.
A compressor can be partially unloaded and its output reduced by increasing the clearance volume. Clearance bottles connected to compressor cylinders via valves are often provided for this purpose. The Owsley et al. U.S. Pat. No. 4,737,080, which is incorporated herein by reference, discloses an unloader of this type wherein the valve members are controlled by means of a pilot valve. The pilot valve may be positioned so as to either apply suction line pressure to the valve members, holding them in the closed position and thereby loading the compressor, or to vent the valve assembly, allowing the valve members to open and thereby partially unload the compressor.
A problem with this type of clearance bottle unloader system is that the valves are required to be held fully open or fully closed for continuous operation in one mode or the other, and cannot be used in a variable manner that cycles with each revolution of the compressor.
The Sperry U.S. Pat. No. 5,695,325, which is incorporated herein by reference, discloses an unloader system wherein the compressor may be unloaded in steps during operation by rotating a valve guard mounting the valve members in synchronization with the compressor crankshaft. This is accomplished using a stepper motor keyed to the compressor""s crankshaft position to actuate a radial unloader valve assembly. While this arrangement does allow the compressor to be loaded and unloaded with each revolution of the compressor crankshaft, the mechanism is not suited for every compressor unloading application.
The present invention relates to pneumatically loading and unloading a reciprocating compressor in a smooth, stepless manner with each revolution of the crankshaft. This is accomplished by using a controlled pressure to hold the unloader valves closed until the compressor piston reaches the desired position in its cycle. By adjusting the set point of a pressure regulator, the effective use of any shape and size of clearance cavity can be smoothly varied from zero impact to full impact.
Heretofore there has not been a compressor unloader system available with the advantages and features of the present invention.
In the practice of the present invention, an unloader system is provided for a reciprocating gas compressor having a cylinder, a piston reciprocably mounted in the cylinder, a rotatable crankshaft connected to the piston, a suction valve assembly, and a discharge valve assembly for selectively communicating suction and discharge lines with the compressor cylinder. An unloader valve assembly including a valve seat structure, a valve guard, and multiple poppet valve members is provided to allow selective communication between the compressor cylinder and a clearance bottle. The opening and closing of the unloader valve assembly is controlled by manipulating a control pressure acting through a manifold against the stem ends of the poppet valve members by means of a pressure regulator connected in series with a pressure source. When the pressure in the compressor cylinder acting on the heads of the poppet valve members exceeds the control pressure acting on the stems, the poppet valve members open, partially unloading the compressor.
The principal objects and advantages of the present invention include: providing an unloader system for a gas compressor; providing such an unloader system which operates in a relatively smooth, stepless manner to vary compressor loading with clearance pockets; providing such an unloader system which makes partial use of fixed clearance volumes; providing such an unloader system which provides essentially infinite unloading capabilities; providing such an unloader system which can cycle with each revolution of the compressor crankshaft without reference to the crankshaft position; providing such an unloader system where the set point at which the clearance cavity is opened is quickly and easily adjustable; providing such an unloader system which can be utilized with various types of valve assemblies; providing such an unloader system which is economical to manufacture, efficient in operation, capable of long operating life and particularly well-adapted for the proposed usage thereof.